The present disclosure relates to power amplification circuits.
A power amplification circuit is used in a mobile communication device such as a cellular phone in order to amplify the power of a radio frequency (RF) signal to be transmitted to a base station. A bias circuit is used in such a power amplification circuit. The bias circuit is for supplying a bias current or bias voltage to a power amplification transistor. For example, in Japanese Unexamined Patent Application Publication No. 2005-228196, a bias circuit is disclosed that is formed of diode-connected transistors TR2 and TR3, a rectifying emitter-follower transistor TR4 and a constant-current-source transistor TR5.
Gain expansion may occur in a power amplification circuit at the time of intermediate power output. Since the linearity of the gain of the power amplification circuit is lost due to the occurrence of the gain expansion, the linearity of the power amplification circuit is degraded. In order to deal with this problem, for example, a configuration has been considered in which the transistor TR5, which operates as a constant current source, is provided between the emitter of the transistor TR4 and a reference potential, as in the bias circuit disclosed in Japanese Unexamined Patent Application Publication No. 2005-228196. Thus, variations in the emitter voltage of the transistor TR4 that occur as the amplitude of a high-frequency signal increases are suppressed. In addition, increases in the average value of the bias current at the time of intermediate power output are also suppressed. Thus, gain expansion of the high-frequency amplification transistor TR1 is suppressed and the linearity of the power amplification circuit is improved. However, in the bias circuit, since the transistor TR5 is always in an on state even at the time of small signal input, a constant amount of current flows to the transistor TR5. Therefore, there is a problem in that the power addition efficiency at the time of a small input signal decreases because the current consumption of the bias circuit does not decrease in spite of the current consumption of the transistor TR1 falling at the time of a small signal input. Here, the term “gain expansion”, refers to an increase in gain that occurs with an increase in output power in an intermediate power output region.